The present invention relates to groundwater velocity probes, and, more particularly, groundwater velocity probes for measuring speed and direction of groundwater flow in an aquifer.
Unsuitable management and contamination of groundwater resources over the past century has damaged a substantial number of aquifer systems, some beyond repair. Contamination resulting from the use, storage, and disposal of hazardous material needs to be tracked and often removed from groundwater systems. This requires an understanding of the mechanism governing the transport of contaminants in the subsurface and will ultimately demand accurate predictions of the fate of contamination. An essential piece of information in assessing a contaminant""s fate is the groundwater velocity. Advection, defined as the component of solute movement attributed to transport by the flowing groundwater, is usually the dominant factor in the migration of dissolved contaminants in aquifers. Advection of a contaminant occurs at an equal rate to the average linear groundwater flow velocity (neglecting dispersion, diffusion and assuming a conservative solute). Therefore, accurately measuring the average linear flow velocity is key in predicting the rate of contaminant transport in a groundwater system.
The U.S. Environmental Protection Agency (EPA) has established criteria for determining the groundwater vulnerability at a hazardous waste facility based on the groundwater velocity. These criteria require the calculation of the time of travel of groundwater along a 100-foot flowline originating at the base of the hazardous waste unit. The site is defined as having vulnerable hydrogeology if the groundwater velocity exceeds this distance within a time period of 100 years. It becomes clear that the importance of accurately determining the average linear velocity of the groundwater is essential in determining a site""s compliance with government regulatory guidelines.
Similarly, in Canada, the Canadian Ministry of Environment and Energy reserves the authority to permit the use of property for contaminant attenuation or treatment. It must be assured, however, that discharge into neighboring property has no more than a negligible effect on the existing and potential reasonable use of this property. Identifying contaminant discharge into a neighboring area requires the calculation of its rate of transport. The need to identify the average linear groundwater velocity becomes clear.
A substantial number of field methods have been developed to measure average linear velocity. These methods are divided into two broad categories: indirect methods and direct methods.
Indirect methods require measurement of hydraulic conductivity or transmissivity, estimation of the effective porosity and measurement of the hydraulic gradient. Applying Darey""s Law, the average linear velocity can then be calculated as shown below. Indirect methods include bail tests, pumping tests, and mapping of hydraulic heads.   ν  =                    (        K        )                    (        θ        )              ⁢          xe2x80x83        ⁢                  (                  Δ          ⁢                      xe2x80x83                    ⁢          H                )                    (                  Δ          ⁢                      xe2x80x83                    ⁢          L                )            
Where,
K=Hydraulic Conductivity (L/T)
H=Hydraulic Head (L)
L=Linear Distance (L)
O=Porosity (dimensionless)
The indirect calculation of the linear groundwater flow is limited by the difficulty in accurately measuring hydraulic conductivity. Hydraulic conductivity quantifies the permeability of a medium and has an associated error range of an order of magnitude.
Empirical relationships exist between hydraulic conductivity, the characteristics of a porous medium, and the properties of a fluid, by means of theoretical and dimensional analyses. The Hazen Formula is a frequently used empirical formula that calculates the hydraulic conductivity of a porous medium on the basis of grain size distributions. The method is applicable to sands with an effective grain-size (d10) between approximately 0.1 and 3.0 mm. The Hazen Formula is outlined below:
Hazen Formula:
K=C(d10)2
Where,
K=Hydraulic Conductivity (L/T)
C=Hazen Coefficient (dimensionless)
d10=Effective Grain Size (L)
The bail test or slug test method involves the imposition of in instantaneous change in hydraulic head at a single well that penetrates a water bearing formation. Either adding (slug test) or removing (bail test) a column of water from the well induces a change in hydraulic head. The rate at which the system returns to its initial equilibrium state is dependent upon the permeability of the formation and the well-bore conditions. By measuring the change in the water level as a function of time, it is possible to compute the hydraulic conductivity of the formation in the immediate vicinity of the well. Generally, several such measurements are made to establish the formation""s range of hydraulic conductivity, and then an average value is used to estimate flow velocities.
The principle disadvantage of an indirect method, such as a slug or bail test, is that hydraulic conductivity is only measured to within an order of magnitude. This means that velocities can only be calculated to within an order of magnitude. Slug and bail tees have the additional disadvantage of being conducted through well screens and filter packs, which can make analysis of the data complex.
Pumping tests involve pumping water from a well for a predetermined period of time, at a fixed rate. The drawdown of the water table is measured at the pumping well and selected observation wells in the vicinity. The data are used to calculate large-scale hydraulic conductivity values, which are then applied to velocity estimation. Unfortunately, pumping test require a considerable investment of time and can be expensive to perform.
In the direct measurement of the groundwater flow velocity, an instrument is inserted into the porous medium or a monitoring well, and is used to measure the rate of groundwater movement. This measurement can be directly related to the average linear flow velocity or related using a calibration constant determined indeperndently. Direct methods include thermal gradient instruments such as the K-V Associates Model 30 Geoflo Meter(copyright), and concentration gradient tests such as a borehole dilution test, and natural gradient tracer tests.
A thermal gradient instrument consists of a, submersible probe attached by cable to a controlling device located at the ground surface. The probe contains a central pair of thermistors. After the probe is lowered to the desired depth in the well, the central heat probe emits a short duration heat pulse. The resulting heated water advects in the direction of the groundwater flow at a rate dependant on the average linear groundwater velocity. The thermistors are monitored at the surface to determine the relative thermal differences of the five opposing thermistor pairs. Based on these relative differences the user can estimate relative groundwater velocity. The K-V Associates Model 30 Geoflo Meter(copyright) exemplifies this method. The Geoflo Meter(copyright) is designed to directly measure the groundwater velocity and flow direction. Since this method is based on relative differences in values of temperature, it must be calibrated against a set of standards. This is accomplished by using a calibration chamber supplied by the manufacturer.
Direct measurement of groundwater velocity and flow direction in this manner is limited by the fact that measurements are made within a well screen. First there may be problems with calibration. If the conditions of the well screen used for calibration are not identical to those in the surrounding aquifer, calibration is likely to be inaccurate. Second, there may be problems with instrument sensitivity because flow velocities in the range of 10 cm/day or less are not easily measurable with precision.
Borehole dilution is a well-established method for analysing groundwater velocity. It is a tracer technique that is performed in a section of a well isolated by inflatable packers from the remainder of the well. A small amount of tracer is quickly injected into the isolated test section and is subjected to continual mixing as groundwater gradually replaces the tracer solution in the well. A log normalized concentration-versus-time curve can be plotted allowing for the magnitude of the horizontal velocity of the groundwater flow to be calculated. Testing vertically distinct sections of the well, a picture of the vertical velocity variation in the aquifer (near the well) can be obtained. The measurement of the lateral variability of the flow system depends on the number and distribution of monitoring wells. This method endeavours to account for the flow system distortions through a well screen. However, this accounting requires a calibration test for each well.
The chief disadvantage of the borehole dilution method is the need for mixing in the well. Downhole mixers have not proven reliable and recirculation of the tracer solution from the well to the surface and back limits the depth at which the measurements can be made. The difficulties associated with calibration for an in-ground well screen are also non-trivial.
The natural gradient tracer test is arguably the most representative method for determining the velocity of a particular solute in a groundwater flow system. The test involves injecting a tracer into a flow system and monitoring its progress with time. Despite its representativeness, this method is a most expensive and costly option. A test can run for years and requires that the test area be well instrumented with groundwater samplers.
The present invention is directed to groundwater velocity probe for measuring groundwater velocity in an aquifer.
According to one aspect of the present invention, a groundwater velocity probe is provided comprising a groundwater velocity probe comprising a surface with a first portion and a second portion, means for injecting a tracer proximate the first portion, first means for detecting the tracer proximate the second portion, and means for measuring time elapsed between injecting the tracer and detecting the tracer. The surface can be cylindrical. The cylindrical surface can further comprise a third portion, and a second means for detecting the tracer can be provided proximate the third portion, such that the means for injecting the tracer is disposed between the first means for detecting and the second means for detecting and such that the first and second means for detecting are oriented along different flowpaths originating from the means for injecting. The probe can further comprise a pointed end for facilitating hammering of the probe into an aquifer. The first and second means for detecting the tracer can include conductivity sensors. The means for detecting the tracer can be disposed along substantially the same plane at which the tracer is injected. The probe can also include first ad second warning detectors disposed above and below the means for detecting the tracer, such that the first and second warning detectors sense non-linear travel of the sensor. The probe can also include angular indicators for enabling rotation of a cylindrical probe about its axis by a non-angular displacement. The probe can also include computing means for estimating groundwater velocity proximate the second portion.
A further aspect of the invention provides a groundwater velocity probe comprising a cylindrical surface with a first portion, a second portion, and a third portion,means for injecting a tracer proximate the first portion, first means for detecting the tracer proximate second portion, second means for detecting the tracer proximate the third portion, and means for measuring time elapsed between injecting the tracer and detecting the tracer, such that the first and the second means for detecting are oriented along a common flowpath originating for the means for injecting.
In another aspect, the present invention provides a method of measuring groundwater velocity in an aquifer comprising the steps of:
(i) providing a groundwater velocity probe in an aquifer, the groundwater velocity probe having a surface with a first portion and a second portion, means for injecting a tracer having a characteristic property proximate the first portion, means for detecting the characteristic property of the tracer proximate the second portion, and means for measuring time elapsed between injecting the tracer and detecting the characteristic property of the tracer,
(ii) injecting the tracer proximate the first portion,
(iii) starting a timer and a datalogger,
(iv) detecting the characteristic property of the tracer proximate the second portion,
(v) inputting an electrical signal representative of the characteristic property to the datalogger, and
(vi) measuring time elapsed between injecting the tracer and detecting the characteristic property with the datalogger.
The method can also include the further step of inputting the time elapsed information into a computer for estimating the velocity of the tracer proximate the second portion.